Internal combustion engines installed in recent vehicles often incorporate a valve characteristic varying mechanism for varying a valve characteristic value of intake valves or exhaust valves. For example, patent document 1 describes an internal combustion engine incorporating a valve timing varying mechanism that varies the opening and closing timings of intake valves and exhaust valve, or the so-called valve timing. Referring to FIG. 10, the valve timing varying mechanism 1 includes a pulley 2 and a rotor 4, which is connected to a camshaft 3 so as to be integrally rotatable with the camshaft 3. The pulley 2 and rotor 4 are coupled to be relatively rotatable. The pulley 2 is connected to and driven by an output shaft of the internal combustion engine by a timing belt, which is not shown in the drawings. The pulley 2 is rotated in synchronism with the output shaft. In FIG. 10, an arrow indicates the rotational direction of the pulley 2. A plurality of vanes 5 extend radially outward from the center of the rotor 4. The pulley 2 includes a plurality of cavities 6, each of which retains a corresponding vane 5. As shown in FIG. 10, in each cavity 6, the corresponding vane 5 defines an advancing hydraulic pressure chamber 7, which is located at the side of the vane 5 that is opposite to the rotational direction of the pulley 2, and a retarding hydraulic pressure chamber 8, which is located at the side of the vane 5 that is in the rotational direction of the pulley 2.
In the valve timing varying mechanism 1, the hydraulic pressure of the hydraulic oil supplied to the advancing hydraulic pressure chambers 7 and the retarding hydraulic pressure chambers 8 is adjusted to vary a relative phase a of the rotor 4 and camshaft 3 relative to the pulley 2. As a result, by varying the rotation phase of the camshaft 3 as the output shaft of the internal combustion engine rotates toward the advancing side or the retarding side as indicated by the arrows shown by broken lines, the valve timings of the intake valves or exhaust valves may be varied to timings suitable for the engine operation state. By varying the valve timing in this manner, the engine output and fuel efficiency may be improved.
The valve timing varying mechanism 1 is driven by hydraulic pressure supplied from an engine-driven hydraulic pressure pump, which is driven using the torque of the engine output shaft. Thus, when the engine speed is low, the amount of hydraulic oil pumped out of the hydraulic pressure pump decreases. This lowers the hydraulic pressure supplied to the hydraulic pressure chambers 7 and 8. Further, when the temperature of the hydraulic oil is high, the viscosity of hydraulic oil becomes low and thereby increases the amount of hydraulic oil leaking out from various parts of the engine. This further lowers the hydraulic pressure of the hydraulic oil supplied from the hydraulic pressure pump. As a result, the hydraulic pressure of the hydraulic oil supplied to the valve timing varying mechanism 1 may become insufficient.
In this manner, when the hydraulic pressure of the hydraulic oil becomes insufficient, the responsiveness of the valve timing varying mechanism 1 is lowered, and variations cannot be accurately made in accordance with changes in the target value of the relative phase α. This may cause hunting and lower the engine performance, which includes engine output and fuel efficiency.
Such problems caused by insufficient hydraulic pressure of the hydraulic oil may occur not only in valve timing varying mechanisms such as that described above but also in other types of valve characteristic varying mechanisms operated by hydraulic pressure supplied from an engine-driven hydraulic pressure pump.
Patent Document 1: Japanese Laid-Open Patent Publication No. 9-324612